Apparatus for determining physical properties of fluids



P 1944- E. G. ASHCRAFTI APPARATUS FOR DETERMINING PHYSICAL PROPERTIES OF FLUIDS ori inal Filed De c. 19, 1939' a: I: Z Z i ll llll Patented Sept. 19, 1944 APPARATUS FOR DETERDHNING PHYSICAL PROPERTIES OF FLUID Ernest G. Asher-aft, Long Beach, Calif.

Continuation of application Serial No. 309,986, December 19, 1939. This application August 13,

1941, Serial No. 406.

7 Claims.

This invention relates generally to apparatus for determining certain physical properties of fluids, and particularly for determining specific gravity. The'present application is a continuation of my prior copending application of the same title, filed December 19, 1939, Ser. No.

It is a general object of the invention to'provide an instrument for determining efficiently and embodiment has been designed for measurement of density or specific gravity of liquids it is to be understood, however, that the underlying principles of the invention are equally applicable to measurement of density of gases, and no limitation to liquids is therefore to be implied.

In accordance with the present invention, there is provided a vibratory liquid coupling element or vane, immersible in the liquid ,to be tested. This vane, whose function is to move a mass of the liquid in which it is immersed, may be regarded broadly as a surface, or pair of oppositely facing surfaces, in contact with the liquid. In accordance with one specific form of the invention, this vane may be in the nature of a substantially rigid, pivoted paddle or blade-like element, though other forms of vane, such as the diaphragm form disclosed in Patent No. 1,570,781 to Ruben, or other modifications, might be substituted. Whenever I hereinafter refer to a vane,v

it will be understood that I employ the expression in a broad and inclusive sense, and that I contemplate broadly any surface of substantial area adapted to contact the liquid, which surface is movable in a direction normal to itself, so as to move a substantial mass of the liquid.

- An alternating accelerative force is applied to this vane in a normal direction. This applied force moves the vane, and with it, a certain volume and mass of the liquid. The acceleration accepted by the vane and themoved mass of liquid, for a given applied force, depends upon the inertia, or mass, of the vane and of the moved liquid, in accordance with Newton's second law of motion. The inertia or mass of the liquid'being 'tional losses.

proportional to the specific gravity of the liquid.

the acceleration so resulting from a given applied force is hence a measure of the specific gravity. Stated in another way, the relationship between the acceleration of the vane and the eifective applied force is a measure of the specific gravity.

The impedance offered by the liquid to the movement of the vane may be divided into two parts or components, that due to the inertia of the liquid, and that due to viscosity or frictional loss in the liquid. v

The inertia component of the impedance offered by the liquid is the equivalent of the effect that would be produced by a mass afixed directly to the vane. This fact underlies and distinguishes the present invention, and will be adverted to later.

The .viscosity .or frictional loss factor is a re sistive force which is subtractive from the accelerative or driving force applied to the vane. Its effect is to reduce the acceleration produced by a given applied-driving force, the effective or resultant driving force available for accelerating the vane becoming the total applied force minus the resistive force owing to viscosity or other fric- I liquid is to be evaluated from the relationship between applied force and acceleration, the ef- Since the specific gravity of the fects of viscosity or frictional loss in the liquid must be reduced to negligible proportions. This is accomplished by the present invention partly in the design of the van'e, which is made of substantial facial area, so as to augment the mass,

vane by an alternating force, preferably sinusoidal in nature, such as may be derived from alternating current power mains. A frequency of cycles per second has been found suitable in prac-, tice.

In accordance with the present invention, the described vane is caused to be driven by, or may be considered as forming a part of, a mechanical vibrator or oscillator, in which forced oscillations are established by driving it through electromag- Y netic means from an alternating current power source. This vibrator or oscillator has a natural resonant frequency or period which depends upon its stiffness, mass, and distribution of mass, and which is varied through a substantial range by the different mass reactions of liquids of different specific gravities in whichthe vane is im-- liquid, and is strictly a measure of specific gravity, may be made to manifest itself and be brought to an indication in various ways. For instance. as one present preferred possibility in accordance with the invention, if the frequency of the driving force be held constant, the variation in resonant frequency accompanying variation in mass reaction on the vane results in either an approach toward, or a recession from, the frequency of the driving force, which in turn means either increase or decrease in amplitude and velocity of vane movement for the given applied driving force. For such conditions, therefore, amplitude or velocity of vane movement may be taken as the measure of the modification of the resonant frequency of the oscillator, and therefore of the specific gravity of the liquid loading the oscillator. Of course, if certain constants of the system were varied, or allowed to vary, the amplitude of vane movement might not indicate specific gravity directly; for instance, variation in strength of driving force would modify the amplitude of vane movement. as would variation in the frequency of the driving force. But the relationship of vane amplitude or velocity to driving force strength, or to driving force frequency, may be taken as measures of the specific gravity.

As one means for bringing the: amplitude or velocity of vane movement to an indication, I may connect thereto a flexible, resilient vibratory reed, which provides an amplified movement that is proportional to the movement of the vane. The indication may be obtained roughly by visual observation of angular deflection of this reed, or may be obtained to greater accuracy by mounting on the reed a coil which cuts a magnetic field and generates a voltage, the latter being easily read on a suitable meter. This particular indication means is but one of many possibilities, however, and the invention is not to be considered as limited to use of the reed as an indication means. 4

r The invention in all its aspects, including further objects, features and advantages, not above mentioned, will be fully understood by now referring to the following detailed description of a present preferred embodiment thereof, reference for this purpose being had to the accompanying drawing, in which:

Fig. 1 is a central longitudinal section of one form of instrument;

Fig. 2 is a central longitudinal section taken on a plane at right angles to the plane of Fig. 1;

Fig. 3 is a diagram illustrating the electrical connections of the instrument; and

Fig. 4 is a detail showing a modified form of reed structure.

In the drawings, numeral [0 designates a suitable casing which may preferably be cylindric as illustrated and closed at its upper end by a cap or plug II and at its lower end by a plug I? which may preferably be screw-threaded into casing lb. The completed casing is made fluid-tight to prevent entry of fluids into it. All of the internal mechanism of the device is preferably mounted on the plug I! so as to facilitate removal from casing I, plug I! thus serving as the stationary mounting means of the mechanism.

Mounted on plug I2 is a fulcrum means for a pivoted mechanical oscillator O. In the particular embodiment of the invention here shown, this fulcrum means includes a pair of supporting blocks II which receive and support, through the intervention of rubber or similar sleeves H, the pivot ends H of a supporting member or mount ii for the vibratory vane V. The central part or saddle |5a of this mount I5 is preferably offset upwardly from the pivots M, for a purpose which will appear hereinafter. fit tightly within the bearing or supporting blocks and tightly around the pivots, so that the mount I5 is thus carried on the mounting structure which allows pivotal movement by reason of the yielding or elastic quality of the mount rather than by relative surface movement. The rubber sleeves I! provide a resilient or elastic compliance, supporting th mount l5 and parts affixed thereto for relative pivotal action on the axis of the pivots l4, and at the same time giving the pivotally mounted system a certain stiffness against pivotal action. Any other suitable pivotal compliance may of course be substituted for the rubber sleeves and pivots, that here shown being simply one illustrative form.

In the particular simple form of the invention here chosen for illustrative purposes, the indication is taken from the instrument by means of a reed .or reeds upstanding from the mount 15. Thus, there are mounted directly on the saddle 15a,- two angle brackets 20, and between these two angle brackets I secure the lower end of the indicator reed or reeds 2|. This reed element may be single; I show it here preferably as double. The reeds normally stand in vertical position and are composed of some suitable elastic or resilient material, as spring steel. Their lower ends are insulated from the brackets 20 and from saddle l5a by the insulation shown at 20a, being best shown in Fig. 4. This insulation not only insulates the reeds from the mounting but also insulates the two reeds 2| from each other.

In the form of device shown in Figs. 1 and 2 the reeds are provided, in the upper portions of their lengths, and on their opposite faces, with armature plates 22, preferably of suitable iron, such as transformer iron. The upper ends of these armature plates 22 are angled outwardly as at 23 to forms. base upon which the small generating coil 24 may be rigidly mounted. This coil is hollow, for purposes which will be hereinafter apparent. The two terminals of coil 24 are electrically connected at 25 with the two reeds, and two lead wires 26 are connected respectively to the two reeds at their lower ends. These are the two lead wires which go to the indicating instrument 3!) which is shown in F.g. 3.

Solidly attached to the saddle l5a and depending therefrom is a rod or beam 3| which extends through the plug l2 and, depending t-herebelow, carries at its lower end the vane V, here shown in the form of a thin blade or paddle 32, and disposed in a plane at right angles to the direction of vibration of the pivoted system.

The rubber sleeves ll respect to plug l2 where it passes through that plug, so as to allow the small freedom which is necessary to allow vibratory movement of vane V and rod 3|. The vertical opening through the plug provides a shoulder 35 against which a rubber packing washer 36 is set by a packing following sleeve 31. The packing washer is compressed longitudinally and is thus expanded radially so as to tightly contact with both the plug I2 and the rod 3|. And it will be noted that the packing washer is located substantially in a plane on the axis of pivots l4, so that it engages rod 3| at a point where the rod movement is at a minimum.

Rod 3| is of such size and stiffness that, within the limits of the bending forces applied to it in operation of the instrument, it is virtually rigid. And vane V is composed of such a material and is of such dimensions as to be light in weight but preferably also rigid. I have found Bakelite to be a suitable substance. Typical dimensions of parts of the instrument are given hereinafter.

Mounted in the upper part of the instrument I provide a magnetic means which performs two functions; both the function of supplying a magnetic field for the polarized reed armatures and the function of providing a magnetic field for the generating coil 24. This magnetic means is preferably in the form of a permanent magnet, here extension 45, the two extensions being spacedly arranged one above the other, as illustrated.

, Generating coil 24 surrounds one of the extensions and a part of the coil winding lies between the two vertically spaced extensions. At their lower ends the pole pieces 44 are mounted upon two supports 46 which in turn are mounted at their lower ends on the plug l2. These supports 46 are also preferably of magnetic material of high permeability, such as soft iron, so thatthey form pole piece extensions which effectively carry the magnetic field down to levels directly opposite the reed armatures 22, the plug l2 being preferably of non-magnetic material.

An exciting coil 50 is mounted upon supports 46 and surrounds the reeds and their armatures. Leads 5| provide a circuit for supplying the exciting coil with alternating current from any suitable source. In the diagram of Fig. 3 a source of such current is indicated. In one form in which my instrument is used, this source of current may be any ordinary single phase alternating current supply, so long as the frequency is comparatively constant and is within a range suitable to the instrument. The coil 50 being excited by an alternating (preferably sinusoidal) 'electric current, of typical commercial power circuit frequency, the reed armatures 22 are alternately polarized in reverse directions, and, .reacting with the fields. of the magnets 4|), cause the reeds, the mount l5 and the vane to be vibrated under the influence of an electromagnetically derived alternating driving force of similar frequency. The vibration of the mount l5 and of the vane may be very small in amplitude, the amplitude of vane movement being typically of the order of a few'thousandths to perhaps a thirty second of an inch, whereas the flexible reeds deflect through substantially amplified angles.

The leads 5| leading to "coil may be extended through a cable 52, along with the leads 26 from generator coil 24, to any suitable points. some uses of my instrument it may be necessary or desirable to use a battery or similar source of power for the instrument. .In that case any suitable battery source, together with any suitable means which produces an alternating current of suitable wave form and suitable frequency, may be used to excite coil 50, and where such ,a

battery source is used it may be carried within,

sired any suitable amplifying or other system may be connected at the indicator circuit, and any suitable form of indicator means may be used.

In Fig. 4 I have shown a modified form of reed and reed armature arrangement. In the form shown in Figs. 1 and 3 the vibratory length of reed is substantially equal to the vertical distance between the mounting brackets 2|] and the lower ends of the armature plates 22 which are fixedly attached, as by riveting, to the reeds. In the form shown in Fig: 4 the reeds 2| are mounted as I have described, but the armature plates 22a are aflixed only at 22b at their upper ends to the upper ends of the reeds, the armature plates diverging slightly outwardly from the reeds. Thus the effective vibrating length of the reeds becomes in this case the vertical distance between the upper edges of mounting brackets 20 and a point near the upper ends of the reeds just below the attachment 22b. The coil 24 is mounted on the upper end of the structure in the in accordance with the invention exhibits a natural reed periodicity of about 56 cycles, and operates satisfactorily on 60 cycle current. In the form of reed shown in Fig. 4,. for such a periodicity, the effective bending length of the reeds is approximately 2%", and the two reeds and the two sets of reed armatures, are each wide and 18 mils thick. To give a further idea of the dimensions and masses of an instrument which is efiicient and accurate in operation, I may state that Figs. 1 and 2 of the drawing are approximately three-quarters scale of a full sized instrument. And in such an instrument I may also state that the vane 32 may be made of Bakelite, 2" thick, and have an area of 1% sq. in., the center of that area being 2 below the axis l4. This arrangement, together with the mass of generating coil 24, which is composed of 600 turns of No. 40 enameled wire of the proportionate coil size illustrated, makes a flexible, resilient vibrating system having a definite natural periodicity in air, which is modified to different degrees by the mass reactions of liquids or diiferent specific gravities.

The exciting coil 58 as here shown may typically consist of about 12,500 turns, have a D. C. re-

sistance of 5000 ohms, and operate on 110 volts 60 cycle alternating current.

From the above detailed description of a specific illustrative embodiment of the invention, it will be seen that I have provided an oscillator, of which the vane V forms a part, so designed that the vane may vibrate in the liquid to be tested. Some part of the oscillator has the property of stiffness (elasticity), and the vibratory parts of the oscillator, together with the quantity of liquid moved by the vane, constitute the overall mass of the vibratory system. The oscillator is driven through electromagnetic coupling from an exciting coil in which flows an alternating driving current of a frequency which, in the present specific embodiment of the invention, is fixed, and may typically be sixty cycles. In the illustrated embodiment, the oscillator derives its stiffness property mainly from the elastic sup-' porting elements If forming the compliance at the pivot axis.

Now this oscillator has one or more natural resonant frequencies which depend upon its stiffness, the stiffness of the compliance, .and upon the mass and mass distribution of its vibratory parts, as well as upon the mass of any liquid that is moved by the vane. A present instrument constructed in accordance with the above description of an illustrative embodiment It is known that a mechanical oscillator of the general class described has a maximum response point (maximum amplitude and velocity) when its natural resonant frequency equals the frequency of the driving force, which in this instance is the frequency of the alternating current flowing in-the exciting coil. Hence, for a given driving frequency, and an instrument of given mass and stiffness constants, a liquid of some particular specific gravity will cause the resonant frequency of the system to equal the driving frequency, and therefore cause a vane deflection of maximum amplitude and velocity; This is the condition of resonance.

At resonance, the mass reactance owing to the mass of the oscillator plus the mass of the liquid moved by the paddle is Just balanced by the elasticity reactance owing to the compliance, i. e., elastic supports, and to other elastic parts of the oscillator. As the vibrating vane travels in either direction past center the kinetic energy of the moving mass, including the mass of the moved liquid, goes to strain the elastic parts of the oscillator, and when the vane then reverses direction, the potential energy so stored in the elastic parts is returned and exerts a force to accelerate the mass, the frictional loss in the resilient mounting such as a rubber or a spring mounting, being small and relatively unimportant. And there is just enough energy so stored in the elastic parts to take care of the acceleration of the mass of the system, including the mass of the moved liquid, so that the driving ing coil has only the work of overcoming the non-reactive or frictional resistances offered by the liquid.

In other words, the mechanical impedance has a defined minimum, and the amplitude and velocity of vane movement experience a corresponding peak, the relations being as follows:

Driving force Impedance The foregoing states the situation as it would exist with the oscillator so loaded that it has become resonant with the driving frequency, though the present instrument in practice, is preferably so designed that the various liquid loads put upon it vary its natural period through an operating range of frequencies to one side of the driving frequency. And when the mass loading of the vane by the liquid is not such as to produce the described condition of resonance, the period of the liquid loaded vibratory system will be some frequency not equal to the period of the driving force, the mass and elasticity reactances will not be in balance, and the mechanical impedance will accordingly become greater. The amplitude and velocity of the vane are hence less than at resonance. In this way, the amplitude and velocity of the vane vary with and constitute a measure of themass loading of the vane by the liquid, and are therefore a measure of specific gravity.

From the above it will be seen that the amplitude and velocity of the vane depends upon the reactive mass loading component of the impedance into which the oscillator works, increasing as that component becomes more nearly equal to, and more nearly balances, the reactance that is due to the stiffness or elasticity of the system. Thus, in summary, increments of mass load on the oscillator vary the natural period of the oscillator, and the oscillation amplitude varies in consonance with the resulting change in differential between the period of the oscillator and the period of the driving force, and hence indicates the change in period, as well as the mass loading of the oscillator, or in other words, the specific gravity of the liquid.

The change in amplitude and velocity of vane Vane velocity= movement with liquids of increasing specific gravities may be either an increase or a decrease, depending upon whether the natural controlling period of the oscillator unloaded, or with light liquid load, is above or below the period of the driving force. An instrument constructed in accordance with the above given specifications has, as before stated, a fundamental periodicity in air of about 45 cycles, and a second harmonic periodicity in air of about cycles. Operating with a driving frequency of 60 cycles, the second harmonic is found to dominate, and as the vane is immersed in liquids of increasing specific gravities, the approach of the resulting decreasing second harmonic periodicity toward the-fixed driving frequency causes an approach toward a condition of resonance, so that vane amplitude and velocity increase.

The amplitude of vane movement of a present actual embodiment of the invention increases from a minimum of a few thousandths of an inch to a maximum of approximately one-thirty second of an inch when immersed in liquids of increasing specific gravities. However, as will be evident, increase in vane amplitude and velocity with increasing specific gravities cannot be considered as a limiting condition, since merely tuning the oscillator to a dominant natural period which is below the driving frequency will reverse thebehavior, causingdecreases in vane amplitude and velocity with increases in specific gravity. I

The variation in amplitude and velocity of ample, it is known that a mechanical oscillator driven from acoil by electromagnetic coupling behaves exactly like a net-work of inductance, capacitance and resistance in circuit with the coil. And changes in the mass or resistance components of the mechanical impedance into which the oscillator works bring about changes in the currents fiowing in the circuit feeding the made large by the rapid vibration of the vane.

' And, of course, viscosity or other frictional losses,

coil which are exactly the same as though corresponding changes in inductive reactance and electrical resistance were made in the equivalent net work. Accordingly, changes in current flow in the circuit of the driving coil, or the voltage across said coil,- are indicative of changes in the mass reactance of the liquid load on the oscillator, or in other words, of the specific gravity of the liquid. Also, there is a phase shift between the alternating driving force for the vane and vane velocity as resonance is approached, which is indicative of the mass reactance of the liquid load, and therefore of specific gravity. The corresponding phase shift between voltage and indicative of variations in specific gravity. All these efiects vary in consonance with vane am plitude and velocity, and a measurement of any one might be made though in any given instance the indication method must of course be appropriate to the physical constants of the instrument.

The present simple illustrative embodiment emascertained by visually observing theangular deflection of the reeds, a more accurate indication is obtained by reading the voltage or current generated by the coil 24 carried by the reeds. Since increase in the amplitude of reed vibration, accompanying increase in amplitude of vane vibration, increases the speed with which generating coil 24 cuts the lines of force between pole extensions 45, the voltage generated by the coil is proportionately increased in like degree, and this voltage, or the resulting current flow, may be read by a meter 30.

Variations in viscosity affect the instrument to a negligible extent. Thisappears to be due, at least in part, to the fact that in moving the liquid the vane moves only a short distance and ofi'ers only small peripheral edge surfaces in frictional contact of the liquid, while it moves a comparativel large mass of the liquid. The effect of mass current fiow in the exciting coil are similarly I ing means, vibratory means connected to the pivto whatever relative extent they remain present, have in any event no objectionable effect at all on the resonant frequency of the oscillator, variation of which with changing specific gravity i the fundamental effect relied on by the present invention.

It will be understood that the drawing and description are to be taken as merely illustrative of and not restrictive on the invention, since various changes in design, structure and arrangement may be made without departing from the spirit and scope of the invention or of the appended claims.

Iclaim:

1. In a device of the character described, the combination of an elastically pivoted beam, 9.

' vane on the free end'of said beam having a fluid contacting surface which moves in a direction normal to itself when said beam rock on its pivot axis, means for applying vibratory energy to said beam to cause it and the vane to vibrate in said normal direction, and means for giving an indication that varies in consonance with vane amplitude.

2. In a device of the character described, vibratory means embodying a fiuid immersible vane, having a relatively broad fluid contacting surface, mounting means for mounting said vibratory means for movement in a direction such that said vane travels substantially at right angles-to its said fluid contacting surface, a flexible, resilient vibratory member connected with said vibratory means adapted to fiex in the direction of vibratory movement of said vibratory means, means for applying vibratory energy to said vibratory means at a frequency corresponding substantially to the natural period of said flexible, resilient vibratory member, whereby said member will vibrate with, said vibratory means but at magnified amplitude, and means for indicating the amplitude of vibra-' tion of the vibratory member.

3. In. a device of the character described, the combination of: supporting means, a resilient vibratory reed pivotally mounted on said supportoted end portion of said reed, said vibratory means including a van immersible in a fluid and having a substantial fiuid contacting surface which travels normally to itself with vibration of said vibratory means, means for vibrating said reed at a frequency corresponding approximately to its natural vibratory period, and means for indicating the amplitude of vibration of said reed.

4. In a device of the character described, the combination of supporting means, a reed mounting pivotally mounted on said supporting means, a resilient vibratory reed mounted on said mounting, vibratory means connected to said reed mounting, said vibratory means including a vane immersible in a fiuid and having a substantial fluid contacting surface which travels normally to itself with vibration of said vibratory means, means for vibrating said reed at a frequency approximately corresponding to its natural vibratory period, and means for indicating the amplitude of vibration of the reed comprising a coil carried by the free end of said reed, means establishing a magnetic field cut by said cell as the reed vibrates, and an indicating instrument fed by the output current of said coil.

loading as compared with frictional losses is also 7 5. In a device of the character described, the combination of supporting means, a reed mounting elastically pivotally mounted on said supporting means, a resilient vibratory reed mounted on said mounting, vibratory means connected to said 6. In a device of the character described, thecombination of: supporting means, a reed mounting pivotally mounted on aid supporting means, a resilient vibratory reed mounted on said mounting, vibratory means connected to said reed mounting having a substantial fluid contacting surface immersible in a fluid and movable normally to itself with pivotal movement of said reed mounting, a magnetic armature carried by said vibratory reed, an exciting coil surrounding said reed'and adapted to polarize said armature, said exciting coil being adapted to be excited by an alternating current of frequency approximately corresponding to the natural vibratory period of the reed, means creating a magnetic field for the polarized reed armature, in a direction transversely of the reed, and means for indicating the amplitude of reed vibration.

7. I11 a device of the character described, the combination of supporting means, a reed mounting pivotally mounted on said supporting means, a resilient vibratory reed mounted on said mounting, vibratory means connected to said reed mounting having a substantial fluid contacting surface immersible in a fluid and movable normally to itself with pivotal movement of said reed mounting, a magnetic armature on said vibratory reed, an exciting cofl surrounding said reed and adapted to polarize said armature, said exciting coil being adapted'to be excited by an alternating current of frequency approximately corresponding to the natural vibratory period of the reed, means creating a magnetic field for the polarized reed armature, in a direction transversely of the reed, and means for generating an electric current of a voltage which is a function of the amplitude of reed vibration, embodying a coil carried by the reed, and means establishing a magnetic field cut by said coil as the reed vibrates.

. ERNEST G. ASHCRAFT. 

